The present invention concerns devices and methods for detecting electromagnetic radiation, in particular, for detecting an energy, or power, density of electromagnetic radiation.
Electromagnetic waves may be detected by help of a so-called bolometer, for example. A bolometer includes a radiation sensor which is able to detect radiated energy, or power, density of mostly weak light, infrared, ultraviolet or microwave sources by registration of a heating of the radiation sensor which occurs by absorption.
A schematic construction of a bolometer is shown in FIG. 1.
Bolometer 10 comprises a radiation sensor 12 including an absorber attached on a substrate 14. Normally, an isolation layer 16 is located between the absorber and the substrate 14 for thermal isolation of the absorber from substrate 14. Here, the thermal isolation may be made by a vacuum, for example, that is, the absorber is arranged at a distance d from the substrate 14. This may be realized, for example, by hanging the absorber over the substrate 14 by spacers which at the same time may function as electrodes. As indicated in FIG. 1, additional isolation material 16 may be provided between the absorber and substrate 14 to achieve a particular thermal isolation. The temperature of the absorber increases with respect to the substrate 14 by incoming electromagnetic radiation 18. Then, changes in temperature of the absorber and, thus, indirectly the amount of radiation arrived are detected by a heat detection structure of the radiation sensor 12. For example, a diode or a temperature-dependent resistor changing the current/voltage characteristic by heating the absorber is used in cooperation with the absorber 12.
Electromagnetic radiation sources may be astronomical objects, for example. A substantial feature of a bolometer compared to other radiation detectors, such as photocells or photodiodes, consists in a wide-band reception characteristic as well as a possibility for detecting radiation which is not, or only hardly, detectable, such as remote infrared or terahertz radiation.
Depending on the wavelength of a radiation source to be examined as well as the reaction time and sensitivity of a bolometer, different radiation sensors, or absorbers 12, are utilized. For example, thin, free-hanging, absorbing metal bands, free-hanging small thermistors, thin-layer structures for short reaction times or superconductive sensors for very high sensitivities are common.
The heat effect caused by the electromagnetic radiation 18 changes a temperature-dependent ohmic resistance R(T) of the sensor, or absorber 12, for example. The resistance R(T) may be measured at a voltage present on the absorber by means of a current measuring device, for example. Thus, conclusions concerning the power density of the absorbed electromagnetic radiation 18 may be drawn. The temperature-dependent ohmic resistance R(T) of a resistor cooperating with the absorber or thermally coupled thereto may generally be described according toR(T)=R0·(1+α(T−T0)),  (1)wherein R0 denotes a nominal resistance at a nominal temperature T0. α designates a temperature coefficient of the temperature-dependent resistance R(T).
When fabricating sensors, or absorbers 12, tolerances generally occur both in the nominal resistance R0 and the temperature coefficient α. For this reason, the sensors 12 have to be calibrated. Particularly with imaging systems with a plurality of pixels, this leads to large calibration tables which have to be stored.
An arrangement of a plurality of bolometers 10 in a matrix for an imaging system is shown by way of example in FIG. 2.
FIG. 2 exemplarily illustrates a 3×3 matrix of bolometers 10, wherein each bolometer 10 corresponds to an image point, or pixel. A particular bolometer 10, i.e. a particular pixel, may be controlled, or readout, in the system illustrated in FIG. 2 by selecting a column and a row of the matrix.
With absorbers, or sensors 12, the temperature T of which is not held constant to save temperature regulation, for example, there are own calibration data for different temperature ranges, e.g. at a distance of some degree Celsius. In imaging systems, these data have to be individually ascertained and stored for each bolometer 10 by a calibration.
In many cases, it is not sufficient to correct images ascertained with an imaging system (such as exemplarily illustrated in FIG. 2) only by help of the calibration data. Rather, an offset balancing has additionally to be made on a temporally periodical basis. For this, a bolometer 10 is covered, for example, by sluing a plate, a so-called shutter, in front of the bolometer 10 so as to shield it from the electromagnetic radiation. For example, all bolometers are intermittently covered by the shutter. In particular, reference images may be generated, from which the offset (e.g. the nominal resistance R0) may be determined. During this time, no image detection may be made.
In some cases, in bolometers, absorbers are also used in connection with thermally coupled diodes. In this context, a temperature-dependence of the forward, or diode, voltage UD according to
                                          U            D                    ⁡                      (            T            )                          =                                            kT              q                        ·            ln                    ⁢                                                    I                D                            ⁡                              (                T                )                                                                    I                S                            ⁡                              (                T                )                                                                        (        2        )            is utilized, wherein T denotes the diode's temperature coupled to the absorber, k=1.38*10−23 J/K denotes the Boltzmann constant, q denotes the elementary charge, ID(T) denotes a temperature-dependent diode current and IS(T) denotes a temperature-dependent diode reverse current. The diode reverse current IS(T), that is, the current through a reverse-operated diode, depends on the fabrication and causes an offset voltage which has to be compensated. As a rule, this is made by calibration.
Radiation detectors, or bolometers, in which no calibration, or offset measurement, or less calibration effort is needed, would be desirable.